In cellular communication systems, the area to be served is divided into cells, each served by a base station, often called base transceiver station (BTS). The base station and user terminals communicate with each other by means' of radio links established therebetween. Time division multiplex access (TDMA) is employed to divide the radio link resource, possibly in combination with code division multiplex access (CDMA), by temporally dividing a given frequency channel into timeslots which are organised in TDMA frames.
Communication from base station to user station is termed downlink communication, whereas communication from user station to base station is termed uplink communication. In TDMA cellular communication systems (e.g. GSM (Global System for Mobile Communications) systems) and combined TDMA/CDMA cellular communication systems (e.g. UMTS (Universal Mobile Telephone Standard, an example of a so-called third generation, or 3G, system) systems), time division duplex (TDD) is employed to divide allocation of signals for uplink transmission and downlink transmission, i.e. for each consecutive TDMA frame of a given frequency channel, some timeslots are allocated to uplink communication, and some are allocated to downlink communication.
The deployment of cells conforming to the Third Generation Partnership Project (3GPP)/UMTS time division duplex (TD-CDMA) mode of operation usually assumes that large groups of cells (and in the limit the whole network) co-ordinate the split of uplink and downlink assigned slots so that the switching points in time (uplink to downlink or vice versa) are the same across this group of cells. Without this, near-located cells could severely interfere with each other because uplink and downlink data transfer would be attempted at the same time on the same frequency and timeslot. The different scrambling (and possibly spreading) codes used are not sufficiently long to reduce the resultant interference to acceptable levels. Therefore, without some degree of coordination of switching points, the overall capacity of the system would be substantially reduced by this otherwise only partially mitigated interference.
However, this significantly reduces the ability of the network to respond to widely varying demands for uplink and downlink rates, both on a request-by-request basis and over all near-concurrent service requests. For example, assume a given switching arrangement across a group of cells, and assume an incoming service request that is best met by altering the slot allocations. This alteration would have to be co-ordinated across the whole group of cells to properly service the request. On the other hand, alteration of the switching points could compromise other ongoing service support within the group of cells.
There arises competing requirements between, on the one hand, co-ordination of uplink and downlink timeslot allocation in nearby cells, and on the other hand, flexibility to allocate different proportions of timeslots between uplink and downlink in a particular cell at an instant in time due to service variation.
One known way of alleviating the disadvantages of this competing requirement is disclosed by U.S. Pat. No. 5,828,948. This suggests an allocation of uplink and downlink time-slots based on the measurements of interference in the uplink and downlink directions. This technique is suitable for TDD cellular mobile communication systems, but imposes additional complexity on the receivers.
Adaptive antenna techniques (beam-forming) can also be used to reduce interference by attempting to direct beams such that the signals associated with users in adjacent cells using the same timeslot, (one for uplink transfers, the other for downlink), interact less than otherwise. This is a complex and costly approach.
In another known approach, cell parameter hopping is defined for TD-CDMA where the scrambling and spreading codes allocated to users can be switched on some basis. This technique is intended to equalize performance between users, since the performance of the different codes is not the same. Therefore this technique shares the bad and good codes between users on a fairer basis. However, this technique does not solve the timeslot clash problem between nearby cells.
It would therefore be advantageous to provide a simpler way of alleviating the above-described problems without, for example, requiring measurements of interference on different time-slots.